A siren emitting a sound of frequency 800 Hz moves away from an observer towards a cliff at a speed of `15ms^-1`. Then the frequency of sound that the observer hears in the echo reflected from the cliff is (Take velocity of sound in air `=330ms^-1`)

Options

A. 838 Hz
B. 885 Hz
C. 765 Hz
D. 800 Hz

Explanation

Here, frequency of sound emitted by siren, `upsilon_(0)=800Hz`
Speed of source, `v_(s)=15ms^(-1)`
Speed of sound in air, `v=330ms^(-1)`
Apparent frequency of sound at the cliff = frequency heard by observer = `upsilon`
Using Doppler.s effect of sound
`upsilon=((v)/(v-v_(s)))upsilon_(0)=(330)/(330-15)xx800~~838Hz`